The present invention relates to a wire vibration prevention mechanism used in a single crystal pulling apparatus.
A single crystal pulling apparatus is designed to grow and pull up a single crystal rod (ingot) from a melt of a polycrystal substance based on the CZ method (Czochralski method). A conventional single crystal pulling apparatus consists mainly of a main chamber, and in this chamber are a crucible for containing the polycrystal substance (raw material) to be crystallized, a heater surrounding the crucible to melt down the polycrystal substance, a thermal insulator surrounding the heater, etc. The crucible is fixedly mounted on a vertical shaft which is adapted to rotate about its axis. Above the main chamber is provided a pull chamber from which the single crystal ingot pulled up from the main chamber is removed.
In the conventional single crystal pulling apparatus, the as-grown single crystal ingot is connected to and pulled up by a vertical shaft (pull shaft) which is in coaxial alignment with the crucible, and which is adapted to ascend vertically in the pull chamber. More exactly, a seed crystal held at the lower end of the pull shaft is dipped in the molten polycrystal substance in the crucible, and the single crystal grows from the seed crystal as the pull shaft is rotated and raised together with the seed crystal. However, in order to improve the yield of the single crystal ingot from which semiconductor chips are manufactured, the current tendency is to increase the size of the single crystal ingot pulled up in the CZ method. This has necessitated enlargement of the single crystal pulling apparatus, especially in the vertical dimension, as well as complication of the apparatus, so that in most of the newly designed apparatuses the rigid pull shaft is replaced by a wire which can be rolled up on a reel. In this way it is possible to avoid enlargement of the conventional pulling apparatus.
However, since the wire is flexible and tensed in use, it is liable to vibrate in response to an external driving force, especially to an external periodic driving force such as the rotational torque given by a drive means to rotate the wire. The amplitude of the wire vibration becomes large when the wire resonates as the natural free oscillation frequency (eigenfrequency) of the wire, which varies as a function of the free length of the wire, becomes equal to the rotational frequency (hereinafter "rotation speed") of the wire. The vibrating wire transmits the vibration to the suspended single crystal being grown to thereby cause deformed growth of the crystal and makes it difficult to obtain a dislocation-free single crystal ingot. Also, it is known that the distribution uniformity of electric resistivity and that of oxygen density across a cross section of the single crystal ingot depend on the rotation speed of the wire being driven by the drive means. If the optimum rotation speed of the wire for uniform crystallization is equal to or higher than the starting natural free oscillation frequency of the wire, which increases as the free length of the wire shortens, the undesirable resonance of the wire is unavoidable which occurs when the rotation speed and the natural free oscillation frequency of the wire become equal to each other. Therefore, in the conventional single crystal pulling apparatus in which a pull wire is adopted, a compromise had always to be made such that the pull wire was rotated at a rotation speed lower than the optimum value, so that it was difficult to improve the distribution uniformity of electric resistivity and oxygen density across a cross section of the single crystal ingot.
For this reason, various mechanisms have been proposed for prevention of vibration of the pull wire by restricting or bracing the wire at the middle portion of the free length of the wire. For example, Japanese Provisional Patent Publication (Kokai) No. 58-96591 teaches a mechanism for preventing vibration of a flexible pull means consisting essentially of a guide (restriction means) surrounding the pull shaft at a location above a seed chuck, a guide means for holding the guide in a manner such that the guide can move only in the direction parallel to the pull shaft, and a drive means to cause the guide to move along the guide means. Japanese Utility Model Publication No. 62-26458 teaches a single crystal silicon pulling apparatus consisting essentially of a main chamber, a crucible, and a flexible pull means (e.g., wire) carrying a seed crystal at the lower end, and characterized by further having a support means provided at a height corresponding to the middle portion of the free length of the flexible pull means, and a guide means (restriction means) consisting of a ring body held by said support means in a manner such that the ring body can be detached and raised from the support means and a tubular body fixed in the middle of the ring body via support rods and surrounding the flexible pull means.
However, in these proposed mechanisms for preventing vibration of the pull wire, the wire restriction means for restricting the middle portion of the free length of the wire is not adapted to move horizontally, so that it is not possible to correct the departure (deviation) of the restriction point from the axis of rotation of the crucible, which occurs when the single crystal pulling apparatus is deformed in an operational mode by heat, and which causes the single crystal ingot being pulled up to whirl about the imaginary static position and fail to grow in good shape or in dislocation-free manner.